Well, I found a pair of MK III's

It's an interesting pair of MK 3's to say the least, the amps where modified by Mike Moffat of Schiit audio fame. They sound amazing!Not sure exactly what he did, some of you may. There is no can, but a lot under the hood.

GP49 wrote:Lots of 330V photoflash capacitors, connected in series for 660 working volts.

and that's a good thing... right?

Photoflash capacitors were a popular way to add lots of capacitance in a small space. Since most consumer photoflash tubes operate at 300 volts DC, the capacitors are made at a working voltage of 10% higher, or 330 DC.

For the B+ voltages in amplifiers such as the Mark III this is not enough so they get connected in SERIES (with a voltage equalizing resistor across each one). The resultant capacitance is HALF what's printed on the capacitor. So the two big ones in the photo are the equivalent of 375μF, 660 DC working volts.

The bottoms of my modified Mark II amplifiers have photoflash capacitors in them, too. The original quad cans are disconnected. They are there just to maintain stock appearance at the top of the chassis.

That may be true. In their normal applications, they didn't see 120Hz as put out by a full-wave rectifier, but instead the high-frequency pulsating DC produced by the chopper power supplies in photoflash units. Mine have worked just fine and have been absolutely reliable...not one failure. I've bypassed them with film capacitors, as I would any power supply filter capacitor.